TheNile crocodile is actually two distinct species which are only distantly related, according to new research.

Researchers sequenced the DNA of living Nile crocodiles and museum specimens, as well as several 2,000-year-old mummified crocodiles from ancient Egyptian tombs.

To their surprise, the results showed that the large Nile crocodile (Crocodylus niloticus) of eastern Africa is in fact more closely related to four species of Caribbean crocodile than to its smaller West African cousin, which the researchers have named Crocodylus suchus.

Not closely related

The study came about after a herpetologist from the Wildlife Conservation Society came across six crocodiles in an oasis in Chad. At his guide’s recommendation, he jumped in with them, and was puzzled by their unusually docile behaviour.

A sample from one of the crocodiles was later sequenced by Evon Hekkala of Fordham University in New York and her colleagues.

“I kept on sequencing it because I was convinced I was 100% wrong,” says Hekkala. “It wasn’t even remotely related to the Nile crocodile samples I had been working on.”

Nile crocodile hatching from egg

Centuries-old debate

These findings resolve a centuries-old debate surrounding the classification of the Nile crocodile. The name Crocodylus suchus was originally coined in 1807 by the French naturalist Geoffroy Saint-Hilaire, who considered it to be a subspecies of the Nile crocodile. However, his ideas were not widely accepted.

Interestingly, the ancient Egyptians may have recognised the difference between the two species. All the crocodile mummies the team studied belonged to Crocodylus suchus, and the ancient Egyptians were said to use a small, tamer crocodile in their ceremonies, considering it to be sacred.

Saint-Hilaire referred to this species as the “sacred crocodile”, and the researchers have proposed using this as its common name.

Not a “living fossil”

Hekkala and her colleagues are now working to formally describe the new species. Although crocodiles can be hard to tell apart from their exterior features, preliminary work suggests that the two species have distinct skulls. Research from the 1970s by the leather industry also suggests they may have different scale patterns.

Although the Nile crocodile is often thought of as a “living fossil” that has remained unchanged for millions of years, the new research suggests that this is not the case. Hekkala said, “Our paper shows that the true Nile crocs in east Africa are as young as humans are. C. suchus is only slightly older.”

The research suggests that the two species used to overlap throughout Africa. However, C. suchus later went extinct from suitable habitats in the Nile and retreated into the dry interior of western Africa. Meanwhile, C. niloticus is thought to have spread across the Atlantic to the Caribbean, possibly riding ocean currents that flow from eastern Africa and around the Cape of South Africa.

Important conservation implications

The team’s findings have important implications for the conservation and management of these iconic reptiles.

The newly confirmed species C. suchus is already in decline and its range is contracting due to unregulated trade in its skins and meat. It is also under threat from industries such as oil extraction.

The ‘traditional’ Nile crocodile, C. niloticus, has been viewed as a model for the sustainable use of wildlife, with several nations planning to increase their harvest of crocodile skins. However, the new findings halve the range of this species and so have big implications for its sustainable management.